The present invention relates generally to laser chambers for excimer lasers and other electric discharge lasers and more particular to pulse high voltage feedthrough structures for such chambers.
FIGS. 1A and 1B are cross-sectional views showing the inner structure of a laser chamber 10 in a conventional transversely excited (TE) excimer laser (see Akins et al., U.S. Pat. No. 4,959,840, issued Sep. 25, 1990, and incorporated herein by reference in its entirety). FIGS. 1A and 1B are excerpts from the ""840 patent. FIG. 1C is a cross section similar to FIG. 1B but showing the entire length of a prior art laser chamber. A laser enclosure 10 provides isolation between a laser chamber interior and the exterior. Typically enclosure 100 is formed by upper and lower enclosure members 12 and 14, which are coupled together and sealed using an o-ring seal 16, extending along a perimeter of enclosure 10. The laser chamber interior is filled to a predetermined pressure with a lasing gas mixture including the hazardous gas fluorine, F2. A pulsed electric discharge is generated in the lasing gas mixture in a discharge region 22 by a high voltage pulse applied between a cathode assembly 18 and an anode assembly 20. Since anode assembly 20 is generally electrically grounded to laser enclosure 10, the entire pulse high voltage is applied between cathode assembly 18 and upper enclosure member 12. The pulsed gas discharge typically produces excited fluorine, argon fluoride or krypton fluoride molecules, which generate laser pulse output energy. The pulse output energy propagates from discharge region 22 through an optical output window assembly (not shown in FIG. 1A). Cathode assembly 18 and anode assembly 20, defining discharge region 22, and extend for about 28 inches substantially parallel to one another for most of the length of laser chamber 10 perpendicular to the plane of FIG. 1A.
Recirculation of the lasing gas mixture is provided by a tangential fan 46. As shown by arrows in FIG. 1A, the flow of lasing gas mixture is upward through tangential fan 46 and transversely across discharge region 22 as directed by a vane member 52. The lasing gas mixture that has flowed through discharge region 22 becomes dissociated and heated considerably by the pulsed gas discharge. A gas-to-liquid heat exchanger 58, extending substantially the length of laser chamber 10 perpendicular to the plane of FIG. 1A, is positioned in the gas recirculation path to cool the heated gas. Recirculation cools and recombines the lasing gas mixture, thereby allowing repetitively pulsed laser operation without replacing the lasing gas mixture.
In this prior art chamber high voltage pulses in the range of about 16 kv to 30 kv are applied to cathode 20 at repetition rates of about 1000 pulses per second from a high voltage bus 70 mounted on top of chamber 10 as shown in FIG. 1C. Bus 70 consists of a thin copper plate mounted on a xc2xd thick aluminum plate with rounded surfaces. (This aluminum plate is referred to as a xe2x80x9ccorona platexe2x80x9d since its purpose is to reduce or minimize corona discharge from the high voltage bus.) The bus is energized by a peaking capacitor bank typically consisting of 28 individual capacitors (not shown) mounted in parallel and electrically connected between bus 70 and the metal enclosure 10 which functions as ground. The high voltage pulses are transmitted to cathode 18 through a feedthrough structure consisting primarily of 15 feedthrough conductor assemblies 72 as shown in FIGS 1A, B and C.
Cathode 18 and each of the 15 feedthrough conductors carrying peak voltages in the range of 16 kv to 30 kv must be insulated from the metal surfaces of enclosure 10 which is at ground potential. Because of the corrosive F2 environment inside the chamber only certain high purity ceramic insulators such as high purity A2lO3 can be used for the portion of the feedthrough assemblies exposed to the gas environment.
With a design of the type shown in FIGS. 1A, B and C ceramic parts 28 are sandwiched in between a brass part 32 and an aluminum part 12. The laser chamber is subject to temperature swings between normal ambient temperature of about 23xc2x0 C. and temperature of about 120xc2x0 C. The coefficients of thermal expansion of aluminum, brass and A2lO3 are about 23xc3x9710xe2x88x926/xc2x0 C., 20xc3x9710xe2x88x926/xc2x0 C. and 8xc3x9710xe2x88x926/xc2x0 C. respectively. The distance between the two end feedthroughs is about 22 inches. Therefore, in this distance a 100xc2x0 C. temperature increase would produce unrestrained expansions of about 0.052 inch, 0.045 inch and 0.017 inch respectively for aluminum, brass and AlO2. This makes a difference of about {fraction (1/32)} inch between the ceramic and metal parts. It is important that good seals be provided for the feedthrough assemblies to prevent hazardous fluorine from escaping into the working environment.
The issues discussed above have been dealt with in the design of the laser portrayed in FIGS. 1A, B and C. This laser utilizes three main insulators 28A, 28B and 28C to insulate the cathode 18 from the chamber member 12. In this prior art design as shown in FIG. 1C, fifteen feedthrough connectors are separated into three separate groups so that the effective length of the sealed region of each of the resulting metal-ceramic-metal sandwiches is only about 6 inches. This reduces the differential expansion by a factor of about 3.5 as compared to a single piece insulator covering the entire electrode length. Sealing at the feedthroughs is provided by tin-plated, nickel-copper alloy xe2x80x9cCxe2x80x9d seals 32 and 34 as shown in FIGS. 1A and 1B. Seal 32 are circular seals making a seal around each of the 15 feedthroughs at the insulator 28, cathode support 26 interface. Each of three seals 34 make the seal between the bottom of upper chamber 12 and the top of one of the three insulator plates 28, each seal 34 providing a single seal around five feedthroughs.
In this prior art design, cathode support bar 26 is bolted to cathode 18. Threaded feedthrough rod 36 threads into cathode support bar 26. Feedthrough insulator 41 insulates rod 36 and a feedthrough nut (not shown in FIGS. 1A and 1B) is threaded onto feedthrough rod 36 and holds insulator 41 in place. A holddown bolt with a Belleville washer is passed through an insulator cap called a xe2x80x9cbuttercupxe2x80x9d is then screwed into the feedthrough nut to apply a compressive force clamping the electrode support to the top inside wall of the chamber with insulator plate 28 and seals 34 and 32 sandwiched in between.
The prior art feedthrough designs shown in FIGS. 1A, B and C has been commercially very successful and is utilized in hundreds of excimer lasers currently operating around the world. The design is basically trouble-free with extremely minimal problems with leakage or electrical failure despite the harsh F2 environment and in many cases continuous round-the-clock operation for weeks and months at a time.
However, the very large number of parts of the above described prior art design make the fabrication expensive. Also, a need exists for a reduction in the electrical inductance associated with the feedthrough design. Therefore a need exists for a better electrical feedthrough system for electric discharge lasers.
A feedthrough structure of a gas discharge laser chamber conducts electric power through the wall of a sealed gas enclosure to a single piece elongated electrode inside the enclosure. The feedthrough structure includes a single piece integrated main insulator larger than the electrode. The main insulator is compressed between the electrode and the wall of the enclosure. The surfaces forming interfaces between the electrode and the single piece insulator are the insulator and the wall are all very smooth to permit the parts to expand and contract as the chamber temperature varies. The feedthrough structure also provides mechanical support and alignment for the electrode and includes seals to prevent gas leakage around the feedthrough structure.
A plurality of hollow substantially cylindrical feedthrough insulators integral with the integrated main insulator extend from the upper surface of the integrated main insulator, typically in an evenly spaced row. Each integral feedthrough insulator contains a substantially concentric hole that extends through to the lower surface of the integrated main insulator and accommodates a feedthrough electrical conductor. In a preferred embodiment, each substantially cylindrical feedthrough insulator includes a concentric groove around its outer diameter. In some embodiments, the integrated main insulator is made of a ceramic material, typically 99.5 per cent purity alumina.
The integrated main insulator is clamped between a lower electrode and an upper inner wall of the sealed enclosure, such that the integral feedthrough insulators extend through aligned clearance holes in the enclosure wall. Clamping is typically provided by threading the lower end of the feedthrough electrical conductor into the electrode and tensioning the upper end against an insulating xe2x80x9cbuttercupxe2x80x9d structure on the upper outer wall of the sealed enclosure. Relatively constant clamping forces are preferably provided using Belleville washers. To limit relative thermal expansion between the enclosure and the single piece main insulator the insulator is tightly restrained only at its center position permitting relative expansion and contraction only from its center.
Face seals are provided at the interfaces between the polished surfaces of the integrated main insulator and the respective opposing faces of the electrode and the inner wall of the sealed enclosure. These face seals are positioned substantially concentrically surround the holes through the integral feedthrough insulators, thereby sealing against gas leaks. The face seals preferably sit in grooves in the electrode and the inner wall of the sealed enclosure. The face seals preferably are tin-plated copper-nickel substrate xe2x80x9cCxe2x80x9d-seals. Advantageously, these face seals slide compliantly over the polished surfaces of the integrated main insulator, accommodating differential thermal expansion over a temperature range extending to 100xc2x0 C. above ambient without loss of integrity.
In some preferred embodiments where extensive thermal cycling is expected the three seals at both ends of main insulator are comprised of seals comprised of fluorinated polymer such as Viton rubber which is more accommodating to relative expansion and contracting than the ten-cooled metal seals. Other alternative design modifications to accommodate extensive thermal cycling include: